Reactive power compensators are used for voltage control, reactive/active power flow control, transient and steady-state stabilization for improving the operation and functionality of e.g. electrical power transmission and distribution systems.
Thyristor Controlled Reactors (TCRs)/Thyristor Switched Reactors (TSRs) and Thyristor Switched Capacitors (TSCs) have been widely used as inductive and capacitive reactive power branches of Static Var Compensators (SVCs), respectively.
A drawback of the TCR is the amount of harmonics, e.g. 5th, 7th, 11th, 13th, . . . , etc. harmonics, that it generates during steady state operation and also, during unbalanced network voltage, harmonics of 3rd, 9th, . . . etc. order. For the TCR, harmonic filter branches are normally required in order to reduce the harmonic emission to reach performance levels specified by the network operator. Such filter branches are expensive and require large footprint, e.g. in a substation where they are typically placed.
The TSR does not generate low order harmonics, but a drawback of the TSR is that it creates large current transients in the feeding network when switched in or out, causing voltage disturbances. In order to minimize the voltage transients from the TSR, it is often necessary to divide the TSR in smaller sizes and/or use it together with TSC.
The TSC comprises capacitor banks requiring large footprint, and energizing transients of the capacitor banks have to be taken care of. In particular, energizing a second capacitor bank when a first capacitor bank is already energized, i.e. back-to-back switching, requires the frequency and amplitude of voltage oscillations on a busbar to be limited in order not to generate overvoltages.
Nowadays, it is a trend to replace these branches by Voltage Source Converter (VSC) based chain-link technology. The chain-link technology of the VSC is however rather costly to implement, mainly since it entails advanced power electronics switching devices and advanced control technology.
The various power compensation devices thus have their advantages as well as drawbacks and while high performance can be obtained, it comes with high costs, both initial costs and maintenance costs.